This invention relates to a silane functional adhesive composition and to a method of bonding a window to a coated substrate using the adhesive. In a preferred embodiment the adhesive can bond a window into a structure without the need for a primer for the coated substrate and/or the window.
Polyurethane sealant (adhesive) compositions are used for bonding non-porous substrates, such as glass, to nonporous substrates; these are described in U.S. Pat. No. 4,374,237 and U.S. Pat. No. 4,687,533, both incorporated herein by reference. U.S. Pat. No. 4,374,237 describes a polyurethane sealant containing urethane prepolymers which have been further reacted with secondary amine compounds containing two silane groups. U.S. Pat. No. 4,687,533 describes a polyurethane sealant containing urethane prepolymers which contain silane groups which have been prepared by reacting a polyisocyanate having at least three isocyanate groups with less than an equivalent amount of an alkoxysilane having a terminal group containing active hydrogen atoms reactive with isocyanate groups to form an isocyanatosilane having at least two unreacted isocyanate groups. In a second step, the isocyanatosilane is mixed with additional polyisocyanate and the mixture is reacted with a polyol to form a polyurethane prepolymer having terminal isocyanato groups and pendant alkoxysilane groups. EP 856,569 discloses the use of polyoxyalkylene polymer terminated with silanes having hydrolyzable groups bonded thereto blended with a polyoxyalkylene polymer having no cross-linking groups to bond glass to metal, incorporated herein by reference.
When such adhesives are used to bond glass substrates to coated substrates, such as for window installation in vehicle manufacturing, the lap shear strength of the bonded substrate may be less than desirable for safety or structural purposes. Consequently, a separate paint primer comprising a solution of one or more silanes is typically applied to a coated substrate prior to the application of the adhesive in most vehicle assembly operations for bonding the windshield and the rear window to the vehicle. The use of a primer in assembly operations is undesirable in that it introduces an extra step, additional cost, the risk of marring the coated surface if dripped on an undesired location and exposes the assembly line operators to additional chemicals.
U.S. Pat. No. 6,828,403 (incorporated herein by reference) discloses an adhesive composition which comprises a siloxy functional polymer, a dialkyltin carboxylate or alcoholate catalyst and a secondary amino straight chain alkyl trialkoxy silane adhesion promoter. The adhesive bonds to coated substrates without the need for a primer but does not bond to windows without the need for a primer. U.S. Pat. No. 6,649,016 (incorporated herein by reference) discloses an adhesive composition comprising A) one or more polymers having a flexible backbone and silane moieties capable of silanol condensation; B) one or more titanates or zirconates having one or more ligands comprising hydrocarbyl phosphate esters and/or a hydrocarbyl sulfonate esters; and C) an anhydrous strong organic acid which is miscible with the polymer and enhances the bonding of the adhesive to a coated substrate in the absence of a primer having improved long term durability over the adhesives disclosed in U.S. Pat. No. 6,828,403.
Automotive OEMs have demanded that the paint suppliers provide more robust acid resistant paint systems which stand up to harsh environmental conditions. These paint systems are difficult to bond to due to the increased chemical resistance of the paints. One problem with developing an adhesive which bonds to these paint systems on an automobile is that there are several different paint chemistries. The automobile producers also demand an adhesive for bonding glass into a vehicle which cures rapidly so that the vehicle can be moved without the glass falling out or slipping from the desired location. Additionally, it is important that the adhesive demonstrates stability prior to application, and that is does not cure before being applied to bond the window into the vehicle. If the adhesive cures in the delivery system in the auto plant, the assembly time must be shut down while the adhesive delivery system is purged. Such a shut down is very costly for the automotive companies and must be avoided. In addition to the above needs, such adhesives need to demonstrate rapid cure upon exposure to ambient conditions. The adhesive needs to bond, or link up, to the unprimed coated surface as soon as possible so the risk of the window moving from where it is placed is reduced. Automobiles last upwards of 10 years and the adhesive needs to maintain its integrity and hold the window into the vehicle for the life of the vehicle under a variety of difficult conditions. Thus, what is needed is an adhesive which can be applied to the automotive paints, especially difficult to bond to paints, and to bond glass into a vehicle without the need to use a primer for the coated surface, the window surface or both. Desirably the adhesive demonstrates strong adhesion, adhesive strength, rapid cure, fast link-up, good stability and long term durability.